Controlled Volatility Silicone Materials: Development and Analysis of
Ultra Low OutgassingTM Elastomer for Space
Summer L. Sivas, Vincent Malave, Brian Burkitt, Bill Riegler, Roy Johnson, and Rob
Thomaier
NuSil Technology LLC
1050 Cindy Lane
Carpinteria Ca 93013
Presented at SAMPE 2009, May 19-21 in Baltimore, MD.
ABSTRACT
New silicone materials that aim to limit and reduce the potential for contamination while
maintaining essential physical and chemical characteristics for specialized space
applications are continuously being developed and evaluated. Several agencies, such as
NASA, historically recommend < 1.0 % Total Mass Loss (TML) and < 0.1% Collected
Volatile Condensable Material (CVCM) as a screening level for the acceptance or
rejection of a material for space applications. ASTM E 1559 is an additional test method
used to characterize materials by monitoring the outgassing kinetics and identifying the
volatile components of the material. In this paper, we compare a standard controlled
volatility silica reinforced silicone adhesive that has a history of use in aerospace
applications with a newly developed equivalent Ultra Low OutgassingTM version that
exceeds typical ASTM E 595 requirements achieving < 0.1 % TMLs and < 0.01%
CVCMs. We will compare the cured physical properties and monitor the outgassing
profiles of each material based on ASTM E 1559 test method. We will also examine how
different reinforcing and functional fillers can be incorporated into silicone materials
while continuing to achieve low outgassing characteristics.
1. INTRODUCTION
1.1
Purpose
The purpose of this report is to demonstrate the ultra-low outgassing characteristics of
SCV-2585, a newly developed silicone elastomer with specific low modulus
characteristics ─ similar to CV-2289 ─ that also reduces the potential for contamination
by meeting Ultra Low OutgassingTM requirements.
1.2
Situation Background and Overview of Contents
With the large amounts of adhesive/sealants currently used on a spacecraft,
manufacturers must closely monitor their cumulative contaminant levels. By having
materials with lower outgas levels, manufacturers would have the ability to use more
material when necessary for mechanical reasons, and have the ability to choose material
from a broader material range. Materials with lower TML and CVCM values could create
more efficient solar cells. Lower outgassing would produce lower contamination and
extend the life of a cell especially in Low Earth Orbit where equipment is exposed to the
detrimental effects of molecular oxygen.7 The spacecraft would now last longer in space,
therefore displacing the huge cost to build and transport spacecraft over a longer period
of time.
In recent years, several low outgassing materials have been developed that Ultra Low
OutgassingTM requirements (< 0.1 % TML and < 0.01 % CVCM). However, all of these
systems are resin reinforced silicone materials that have limited physical properties.
Engineers at NuSil recognized a need to develop a low modulus silicone elastomer that
also meets Ultra Low OutgassingTM requirements.
To date, CV-2289 is widely used controlled volatility (CV) silicone material for space
applications offered by NuSil Technology. This silicone elastomer is an ideal material for
potting or encapsulating due to exceptional physical and mechanical properties. This
includes high tear strength, low modulus for CTE mismatch, and a broad operating
temperature range required for materials used in extreme environments such as space,
where thermal temperatures can range from -115°C to 300 °C. Nevertheless, a low
modulus silica reinforced elastomeric silicone that meets Ultra Low OutgassingTM
requirements has not previously been achieved.
In a previous study the thermal outgassing characteristics of CV-2289 and SCV2-2590
were extensively compared.1 This report compares the cured physical properties of the
controlled volatility silicone elastomer, CV-2289, with an equivalent Ultra Low
OutgassingTM version, SCV-2585. The outgassing profiles of each material based on
ASTM E 15593 test method are examined in detail. We also look at how different
reinforcing fillers such as silica or silicone resin affect outgassing characteristics.
1.3
Audience
This paper targets engineers in the aerospace, photonics, and laser industries where low
outgassing materials are needed to prevent contamination of sensitive equipment. Some
understanding of ASTM E 5952 and ASTM E 15593,4 test standards is beneficial, but is
not required.
1.4
Background of Silicone Materials for Use in Space
Silicones have been used as adhesives and coatings for over five decades in the
Aerospace industry. They are ideal materials for the protection of electrical components
and assemblies against shock, vibration, moisture, dust, chemicals and other
environmental hazards particularly encountered in space environments. They also have
the ability to maintain elasticity and low modulus over a broad temperature range. A
more detailed description of silicone materials used for space applications can be found
in Appendix A.
A major drawback to using many silicones compounds for space applications is that
silicone materials can outgas from the polymer matrix and cause subsequent
contamination of expensive equipment and devices. Early NASA flights that used
silicones around the space capsule windows and other areas observed an oily residue
caused by low molecular weight species that had not cross-linked into the silicone
polymer matrix and subsequently outgassed and deposited on the shuttle surfaces. Based
on these discoveries, NASA and other space agencies realized the importance of using
low outgassing materials, known as controlled volatility (CV) materials, and recommends
all adhesives used in extraterrestrial environments be tested prior to use in space for
volatile species that may outgas form the material.
ASTM E 5952 is a widely accepted test standard used to screen materials for volatile
content that may outgass from a material in a vacuum or space environment. NASA and
the European Space Agency (ESA) recommend testing low outgassing materials per
ASTM E 595 prior to use in space. A maximum Total Mass Loss (TML) of 1% and
Collected Volatile Condensable Material (CVCM) of 0.1% are base requirements set out
by these agencies. Although a standard for many years, some question whether these
specifications are stringent enough. In response, NuSil Technology developed an Ultra
Low OutgasssingTM line of silicone materials with limits set out at less than 0.1 % TML
and 0.01 % CVCM. The characteristics and outgassing kinetics of many of these
materials were recently discussed at length.1
2. EXPERIMENTATION
2.1
Materials
Samples
Table list the materials that are compared in this study.
Table 1
Silicone Property
CV-2289
SCV-2585
SCV2-2590
Silicone Polymer
Dipheny-dimethyl
Dipheny-dimethyl
Dipheny-dimethyl
Cure Mechanism
Mix Ratio (A:B)
2-part Pt Addition
(1:1)
2-part Pt Addition
(1:1)
2-part Pt Addition
(10:1)
Reinforcing Filler
Silica
Silica
Resin
2.1.1 Processing for Controlled Volatility
The critical contaminating species typically outgassed from silicone materials are
primarily caused by the low molecular weight silicone cyclics and polymers that are not
covalently bound into the silicone matrix. These species are eliminated in NuSil
Controlled Volatility materials to prevent subsequent outgassing and contamination.
A distillation process can remove low molecular weight linears and cyclics from the
polymer formed during the polymerization process. The low molecular weight materials
condense on the cold finger and are separated to a collection vessel. Depending on the
size of equipment and the ultimate use of the polymer, one to multiple passes through the
distillation process can be performed to remove a sufficient amount of low molecular
weight species based on the ultimate requirement of polymer. To produce the Ultra Low
Outgassing materials, extensive processing time is needed to achieve lower levels. See
Appendix A section 6.2 for more details on this process.
2.2
Methods
2.2.1 ASTM E 5952
This test method is used to determine the volatile content of materials when exposed to a
vacuum environment (i.e. space). The two parameters measured are total mass loss
(TML) and the collected volatile condensable materials (CVCM). Water vapor recovery
is an additional parameter that can also be obtained after the completion of the exposures
and measurements required for TML and CVCM.
2.2.2 ASTM E 595 Test Parameters
Each material sample is preconditioned at 50 % relative humidity and ambient
atmosphere for 24 hours. The sample is weighed and loaded into the test chamber, see
Figure 11, within the ASTM E 595 test stand. The sample is then heated to 125°C at less
than 5x10-5 torr for 24 hours. The volatiles that outgass under these conditions escape
through an exit port, and condense on a collector plate maintained at 25 C. Once the test
is complete the samples are removed from the chamber and the collector plate and
samples are then weighed.
2.2.3 Data Analysis
The CVCM is the quantity outgassed from the sample that condenses on the condenser
plate and presented as a percentage calculated from the difference in mass on the
collector plate before and after the test. The percent TML, the percent total mass of the
material outgassed from the initial sample, is calculated from the mass of the sample
measured before and after the test.
After the specimen is weighed to determine the TML, the WVR can be determined. The
specimen is stored at 50% humidity for 24 hours at 25 C to permit sorption of water
vapor. The specimen is then weighed again which is then subtracted by the he mass
determined after vacuum exposure to obtain the WVR.
The standard criteria for low outgas materials is a 1.0 % TML and 0.10 % CVCM. For
the Ultra Low Outgassing materials, the specification requires < 0.1 % TML and < 0.01
% CVCM, as designated by NuSil Technology.
2.2.4 ASTM E 15593
ASTM E 1559 experiments were conducted at OSI laboratories and test reports were
provided. The isothermal outgassing test apparatus is explained in detail by Garret et al.
and will only be discussed here briefly.3,4,5 A schematic of the ASTM E 1559 test stand
is shown in Figure 12, Appendix B. The material sample can range from 0.5 g to 10 g
and is placed in a temperature-controlled effusion cell in a vacuum chamber. All samples
are preconditioned in accordance with ASTM E 5951 unless otherwise specified.
Outgassing flux leaving the effusion cell orifice condenses on four Quartz Crystal
Microbalances (QCMs) that are controlled at selected temperatures. The QCMs and
effusion cell are surrounded by liquid nitrogen shrouds to ensure that the molecular flux
impinging on the QCMs is due only to the sample in the effusion cell. The TML and
outgassing rate from the sample are determined as functions of time from the mass
deposited on an 80 K QCM and normalized with respect to the initial mass of the sample.
The amount of outgassing species that are condensable, VCM, is measured as a function
of time from the mass collected on the 298 K QCM. After the outgassing test is
complete, the QCMs are then heated to 398 K at a rate of 1K/min. As the QCM heats the
deposited material evaporates. The species that evaporate can be analyzed by a
quadrupole mass spectrometer to quantitatively determine the species observed.
2.2.4.1 ASTM E 1559 Sample Preparation
The CV-2289 and SCV-2585 were cured at 150°C for 15 minutes into discs 1.49 inches
diameter by 0.125 inches thick. SCV2-2500 was cured at 65 °C for 4 hours into to discs:
one disc was 1.491 inches diameter by 0.100 inches thick, the other disc was 1.491 inches
diameter by 0.090 inches thick. The surface area is calculated for both faces and the edge
of the disc.
One of the supplied discs of material was placed in the effusion cell as the test sample.
The samples were tested with no additional preconditioning.
2.2.4.2 Test Parameters
The following parameters were set for each sample:
Chamber Pressures :10-8-10-10 torr
View Factor from QCM to sample: 415 cm2
Test Duration: 72 hours
Sample Temperature: 125 °C
QCM Temperatures: 80 K, 160 K, 220 K, and 298 K
QCM Sensitivity: 4.43 x 10-9 g/cm2/Hz
2.2.5 Data Analysis
2.2.5.1 Outgassing Rate
The outgassing rates for species condensable on the warmer QCMs can be calculated
from curve fits to the data. The total outgassing rate from the 80 K QCM then can be
compared to the outgassing rates for species condensable on the warmer QCMs to
determine the rates of very high volatility species (water and solvents) and the rates of the
remaining species (high, medium, and low volatility).
3. RESULTS AND DISCUSSION
3.1
Comparison of Physical Properties
In Table 2 the results for the typical material properties for each sample were measured
according to ASTM protocols, are listed and compared. These properties were measured
at NuSil Technology.
Table 2: Typical Physical Properties Evaluated for Silicone Elastomers
ASTM
Typical Properties
CV-2289
SCV-2585
SCV2-2590
Uncured
Viscosity
50,000 cps
50,000 cps
3,500 cps
Worktime
1 hour
15 min @
150C
30
1 hour
2 hours
15 min @
150°C
36
4 hrs @ 65 °C
Cured
Cure Time
D412
Durometer, Type A
D412
Tensile Strength
650 psi
684 psi
500 psi
D412
350%
302%
85%
525 psi
506 psi
200 psi
E 595
% Elongation
Lap Shear
Strength
% TML
0.52%
0.06%
0.05%
E 595
% WVR
0.04%
0.04%
0.01%
E 595
% CVCM
0.05%
0.01%
0.01%
D1002
40
Currently, the only silicone materials to achieve Ultra Low OutgassingTM requirements
are resin filled systems like SCV2-2590. The primary goal of this study was to develop
an ultra low outgassing platinum cure, diphenyl-dimethyl copolymer, similar to CV2289, for satellite and space application but to achieve TMLs < 0.1% and < 0.01 %
CVCMs similar to SCV2-2590. These goals were accomplished by achieving 0.06
%TML and 0.01% CVCM for SCV-2585. These results are ten times lower than CV2289. Comparison of the physical properties of CV-2289 and SCV2-2585 in Table 1
show that the desired physical properties are not compromised to achieve these
requirements.
3.2
% Total Mass Loss
3.2.1 Comparison of % TML from ASTM E595 and %TML at the end of the
ASTM E 1559 Test
The %Total Mass Loss (TML) of each sample for the results obtained from ASTM E 595
are compared to the results obtained from ASTM E 1559.
%TML
0.60%
% TML E 1559
0.50%
% TML E 595
% TML
0.40%
0.30%
0.20%
0.10%
0.00%
CV-2289
SCV-2585
SCV2-2590
Figure 1: Comparison of % TML for each silicone elastomer and comparing the results from ASTM
E 595 and ASTM E 1559 at the end of the test.
3.2.2 % TML ASTM E 1559
3.2.2.1 Amount of TML Due to Different Outgassed Species
Molecular weight ranges for the species in the different volatility categories can be
estimated based upon engineering experience related to species condensability and mass
spectrometer data. The "very high volatility" group of species in Table 3 is primarily due
to water and solvents. The "high volatility" species most likely have molecular weights of
50 to 200 amu, the "medium volatility" species fall in the 200 to 400 amu range, and the
"low volatility" species have estimated molecular weights above 400 amu.4
Table 3
ASTM E 1559
Silicone Material
SCV-2585
CV-2289
SCV2-2590
Cumulative Amounts of Estimated Volatility
of Different Outgassed Species
Total % TML
at end of test
0.08%
100.00%
0.43%
100.00%
0.07%
100.00%
very high
volatility
0.076%
89.74%
0.09%
21.84%
0.05%
80.15%
high
volatility
0.00%
5.50%
0.19%
43.71%
0.01%
9.45%
medium
volatility
0.0031%
3.69%
0.14%
31.79%
0.01%
9.05%
low
volatility
0.00091%
1.07%
0.01%
2.67%
0.00%
1.35%
The % TML of the Cumulative amounts of estimated volatility (“very high, high,
medium, and low”) of different out gassed species are plotted in Figure 2. The majority
of the outgasssed species for the SCV materials are of high volatility with low probability
of causing contamination.
Cumulative Amounts of Estimated Volatility of Different
Outgassed Species
Very high
0.40%
high
med
% TML
0.30%
low
0.20%
0.10%
0.00%
CV-2289
SCV-2585
SCV2-2590
Figure 2: % TML of the cumulative amount of estimated volatility of different outgassed species
3.2.3 Total Mass Loss as a Function of Test Time
Figure 3 is a plot of the total mass loss of each sample over the 72 hour period. After the
initial loss and compared to CV-2289, SCV2-2590 shows relatively no mass loss over the
entire course of the experiment.
%TML at 125°C
CV-2289
SCV-2585
SCV2-2590
0.50
0.45
0.40
Total Mass Loss (%)
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
8
16
24
32
40
48
56
64
72
Time (hr)
Figure 3: Total Mass Loss of CV-2289, SCV-2585 and SCV2-2590 as a function of time
3.3
% Volatile Condensable Material (ASTM E 1559)
Figure 4 is a plot of the volatile condensable material from the 298 QCM as a function of
test time.
QCR 298 K
SCV-2585
CV-2289
SCV2-2590
0.012
0.010
VCM (%)
0.008
0.006
0.004
0.002
0.000
0
8
16
24
32
40
48
56
64
72
Time (hr)
Figure 4: Volatile Condensable Materials from CV-2289, SCV-2585, and SCV-2590 condensed on the
298 K QCM
3.4
Outgassing Rate
Total outgassing rate data for the sample were calculated by differentiating the data
obtained from the 80 K QCM. The rates of very high volatility species (water and
solvents) and the rates of the remaining species (high, medium, and low volatility). These
data are presented in Table 4.
Table 4
Total Outgassing Rate
end of test
207.5 pg/cm2/s
100%
35.1 pg/cm2/s
100%
108.5 pg/cm2/s
100%
CV-2289
SCV-2585
SCV2-2590
3.5
Cumulative Amounts of Estimated
Volatility of Different Outgassed Species
high, med and low
very high volatility
volatility
2
105.7 pg/cm /s
1,352 pg/cm2/s
51%
49%
2
26.9 pg/cm /s
8.1 pg/cm2/s
77%
23%
2
80.7 pg/cm /s
27.8 pg/cm2/s
74%
26%
Comparison of the Total Outgassing Rate as a Function of Time
Figure 5 shows the total outgassing rate data as a function of test time for CV-2289,
SCV-2585, and SCV2-2590. These outgassing rates are for species condensable at 80 K
and thus would not include certain gases such as nitrogen and oxygen.
Out Gassing Rate (80 K QCM)
CV-2289
SCV2-2590
SCV-2585
1.E-05
Outgassing Rate (g/g/s)
1.E-06
1.E-07
1.E-08
1.E-09
1.E-10
0
8
16
24
32
40
48
56
64
72
Time (hr)
Figure 5: Total Outgassing Rate for CV-2289, SCV-2585, and SCV2-2590 as a Function of Test Time
(Species Condensable on 80 K QCM)
3.5.1 Predicted Outgassing Rate for SCV-2585
As previously discussed,1 outgassing kinetics show that the majority of the sample loss
for both silicone samples occurs in the first few hours of the test then mass is lost at a
steady rate for the remainder of the test. This indicates that there is a large initial burst of
volatiles, most likely concentrated near the surface of the sample. The slow steady
release thereafter can be due to either a steady state diffusion of gaseous molecules
distributed throughout the polymer matrix that migrate out the material at a constant rate.
In Figure 6, the data for the outgassing rate of SCV-2585 shown in Figure 5 were fit
using a general power function:
y = 3*10-9 x -1.1107
R2 = 0.9516
This model provides a conservative analysis of the predicted amount of volatile
components that will be outgassed over a year.6
2
Outgassing rate (g/cm /s)
Outgassing Rate (QCM 80 K)
1.0E-07
1.0E-08
1.0E-09
1.0E-10
1.0E-11
1.0E-12
1.0E-13
1.0E-14
0
60
120
180
240
300
360
Days
Figure 6: Predicted Outgassing Rate for SCV-2585 as a Function of Time Extrapolated for 1 year.
3.6
Desorption Rate at 80 K QCM
The QTGA test data can be used to determine the relative amounts of the species
outgassed. Recall, at the conclusion of the isothermal outgassing test, the 80 K QCM is
heated at increments of 1K/min while the outgassed species that deposit on the surface
evaporate from the crystal. As the temperature of the QCM is increased during QTGA,
the collected species will evaporate from the QCM in order of their relative volatilities.
As the temperature of the QCM increases, the evaporation rate of the species also
increases until it reaches a peak. The slope of the leading edge is characteristic of the
species being volatilized. In Figure 7, the QTGA data for all three samples are plotted
together as evaporation rate from the QCM as a function of QCM temperature.
QTGA Data: Evaporation Rate from the 80 K QCM of the Collected Outgassed
Material as a Function of QCM Temperature
CV-2289
SCV-2585
SCV2-2590
Evaporation Rate from QCM (g/cm2/s)
1.6E-08
1.4E-08
1.2E-08
1.0E-08
8.0E-09
6.0E-09
4.0E-09
2.0E-09
0.0E+00
50
100
150
200
250
300
350
400
QCM Temperature (K)
Figure 7: QTGA Data: Evaporation Rate from the 80 K QCM of the Collected Outgassed Material
as a Function of QCM Temperature
The major outgassing species for CV-2289 are projected to be low molecular weight
siloxanes as discussed previously.1
4. CONCLUSIONS
As devices and processes become more advanced and sensitive to molecular
contamination, more details of characterization of the construction materials must be
obtained. Ultra low outgassing specification requirements of < 0.1% TML and < 0.01%
CVCM can be useful in the overall management of outgassing species. The results from
kinetic outgassing data allow engineers to better predict the levels of contamination,
migration, and deposition once the materials are in space. Achieving these lower levels
does not show to compromise physical properties and thus a broad range of silicone
elastomers with unique and specific properties can be developed. An area of future
research is to develop a ULO silicone elastomer incorporated with functional fillers that
impart thermally or electrically conductive properties.
5. REFERENCES
1. S.L Sivas, B. Riegler, B. Burkitt, and R. Thomaier. “Testing Ultra Low
Outgassing SiliconeTM Materials,” SAMPE Journal, Jan/Feb 2008.
2. ASTM E-595, “Standard Test Method for Total Mass Loss and Collected
Condensable Materials from Outgassing in a Vacuum Environment.”
3. ASTM E 1559, "Standard Test Method for Contamination Outgassing
Characteristics of Spacecraft Materials."
4. J.W. Garrett, A.P.M. Glassford, and J. M. Steakley, "ASTM E1559 Method for
Measuring Material Outgassing/Deposition Kinetics", Journal of the IEST, pp.
19-28, Jan/Feb 1995
5. A.P.M.Glassford and J.W.Garrett, "Characterization of Contamination Generation
Characteristics of Satellite Materials", Final Report WRDC-TR-89-4114, Jun 82 Aug 89
6. Urayama, F. et al., “Modeling of Material Outgassing and Deposition
Phenomena” Proc. of SPIE, 5526, 137-146, 2004.
7. Banks, B.A., de Groh, K.K., Rutledge S.K., Haytas, C.A. “Consequences of
Atomic Oxygen Interaction with silicone contamination on Surfaces in Low Earth
Orbit,” Proc of SPIE, 8784, 62-71, 1999.
6. APPENDIX A
6.1
Silicone Materials for Use in Space
Silicones typically used in space applications are called Silicone Thermosets, which are
comprised of moldable elastomers and adhesives. These types of silicone materials are
made up of low viscosity polymers, reinforcing fillers, and adhesion promoters. Each
component can be modified for a specific process or application.
6.1.1 Polymer Chemistry
The silicone polymers used for the materials discussed in this report are
diphenyldimethylpolysiloxane co-polymers.
Silicone polymers are composed of
repeating R2Si-O units, having no carbon atoms in the backbone, and are named
polysiloxanes. The structure of silicone accounts for its high degree of flexibility. The
siloxane backbone can be formulated with different types of constituent (R) groups
integrated onto the polymer backbone. Typical constituent groups include dimethyl,
methylphenyl, diphenyl, and trifluoropropylmethyl functionality.
The phenyl functionality is useful in applications where optical clarity and refractive
index matching is desired and low concentration of diphenyl groups can broaden the
operating temperature of those materials substantially.
Figure 8: Diphenyl-dimethyl Vinyl Endblock Silicone Co-polymer
6.1.2 Reinforced Silicone Elastomers
Silicone polymers in this study are reinforced to achieve specific physical properties.
Polymers are typically reinforced with silica and/or resin fillers. Reinforcement fillers
such as silica are the most commonly added to a silicone elastomer systems in order to
improve mechanical properties such as tensile, tear, elongation and adhesion. Filler
particles reinforce an elastomer by reducing the mobility of the siloxane chains. The
uniform distribution and the particle surface area available to make contact with the
siloxane chains have the most influence on the physical properties of a reinforced
elastomer. The bond angles of the silicon-oxygen bonds create large amounts of free
volume in silicone elastomers.
Resin reinforced silicone elastomers on the other hand utilize highly branched
polysiloxanes, also called Polysilsesquioxanes, to reinforce the silicone polymer. Silicone
resins reinforce silicone polymers in three general ways: by covalent bonding –
crosslinking of functional groups on the polymer, chain entanglement of resin and
polymer, and through weak non-covalent intermolecular interactions, such as hydrogen
bonding, between the silicone polymer and the surface of the resin. Resin reinforced
silicones are typically low in viscosity, have high optical clarity, high durometer and
modulus (typically have durometer > 40 Type A), high stress at low elongations, and tear
values of < 100 ppi.
6.1.3 Addition Cure Mechanism
The cure chemistry gives a silicone adhesive its cohesive strength. The silicones used in
this study are two-part platinum addition curing silicones. These two-part systems cure
to a rubbery type hardness and often contain reinforcing fillers to improve physical
properties.
Once the polymer is produced and reinforcing filler is added to make a base, it is
separated into two parts. Catalyst is introduced to Part A while a functional crosslinker
and inhibitor is added to part B. The ratio between the components must be balanced to
give optimal properties. Addition cure silicones utilize a platinum complex as a catalyst
to participate in a reaction between a hydride functional siloxane polymer and a vinyl
functional siloxane polymer. The result is an ethyl bridge between the two polymers. The
reaction mechanism is pictured below:
Figure 9
Platinum systems are often cured quickly with heat but can be formulated to cure at low
temperatures and room temperature if necessary. The advantages of these systems
include fast cure and no volatile byproducts allowing these types of systems may be used
in closed environments. Most systems will cure at room temperature; however, some
need heat to cure. The moldable materials can be casted or injection molded into various
configurations.
6.2
Material for Processing for Controlled Volatility Silicones
Ring Opening Polymerization (ROP) is commonly used for commercial production of
these silicone polymers. The process begins with polyorganosiloxane cyclics reacting
with a chain terminating species, or “end blockers,” in the presence of an acid or base
initiator as shown in Figure 2.
divinyltetramethylsiloxane
octamethylyltetracyclosiloxane
Vinyl endblocked
polydimethylsiloxane
Figure 10: Basic Ring Opening Polymerization (ROP) reaction for a vinyl terminated
polydimethylsiloxane.
The product of this polymerization reaction is a mixture of various molecular weights of
cyclics, short chained linear molecules and higher molecular weight polymers where the
concentrations of each species is based on its thermodynamic equilibrium, Figure 3. The
oily substance and fogging associated with silicones are primarily caused by the low
molecular weight species represented in the smaller peak to the left and the lower
molecular weight portion of the larger peak. These species are eliminated in low
outgassing materials to prevent contamination.
Figure 11: Molecular weight distribution of final ROP reaction products of PDMS.
Stripping the polymer via a distillation process removes low molecular weight linears and
cyclics from the polymer formed during the polymerization process. The distillation
apparatus is typically an evacuated chamber with heated walls and a central cooling
finger designed for condensing low molecular weight molecules. Polymer is driven into
the heated chamber and wiped onto the chamber walls. This exposes a thin film of the
polymer to heat under vacuum conditions. The low molecular weight materials condense
on the cold finger and are separated to a collection vessel. Depending on the size of
equipment and the ultimate use of the polymer, one to multiple passes through the
distillation process can be performed to remove a sufficient amount of low molecular
weight species based on the ultimate requirement of polymer. To produce the Ultra Low
Outgassing materials, more low molecular weight linears need to be removed via
distillation. This adds more processing time to achieve these lower levels.